Electronic manufacturers battling product
reliability issues with their designs revealed the need to measure
and quantify how well a product performs during its service life.
Out of these challenges came the mathematical concept, mean time
between failure (MTBF), a repairable system methodology exhibiting
an exponential cumulative distribution function.

The MTBF value is a quantity that expresses the
magnitude of reliability found in a product, is quoted as the key
characteristic upon which engineers compare designs, and has become
a number advertised with electronic products that quantifies the
product’s reliability. The concept supplies the
definitive ability to express ideas in a readily understood manner
giving engineers the ability to compare design choices before
committing resources in people, time, and development costs to
compliance testing.

MTBF is not the average or the minimum time
before a product’s first failure, it can be best defined
by a simple fraction:

An MTBF value is the result of dividing the
power-on hours for a population of products by the total number of
failures in that population. Therefore, the units for an MTBF value
are hours/fail. The unit for a failure rate is fails/hour. For an
MTBF:

MTBF can be expressed as a failure rate, 1
failure in a billion device-hours = FIT (Failures in Time).

MTBFs are not additive — failure rates
are additive. The failure rate of the assembly is the sum of the
failure rates of the components. Empirically based reliability
models use failures rates.

There are several methods to calculate the
failure rate for an assembly. Using the reliability prediction
procedure Telecordia SR-332, several LED driver topologies commonly
found for low power bulb type applications can be compared.[1] The
topologies considered in this article are:

Non-isolated buck topology without electrolytic
capacitorsNon-isolated buck topology with an electrolytic output
capacitorIsolated flyback topology using primary-side
controlIsolated flyback topology using secondary-side control
(optocoupler based feedback)Two-stage with a PFC stage followed
with a dc/dc stage using primary-side control

How useful is it to know a failure rate or MTBF?
First, as a quantity of comparison, it allows a designer to judge
between designs to perform the same task. Second, failure rates can
be calculated using small samples of products and so the
reliability of the product population can be extrapolated. Third,
failure rates are the key variable in warranty repair costs, which
is a factor used to assess the future profitability of a product.
The failure rate for an assembly can be used to decide how large a
repair center is needed, how large it’s staff should be,
and what the repair center’s budgetary requirements will
be.

The method in this LED driver comparison
consists of using similar bill-of-materials (BOM), assuming there
is a low power A19 design, 10-W load — with a key
assumption that similar components are used across all of the
compared LED topologies. In other words, the compared topologies
use the same MOSFETs, transformers or inductors, EMC filters, etc.
in order to normalize the BOMs to generate comparison failure
rates. This normalization removes a designer’s ability to
use component selection to tweak a design’s failure
rate.

With similar components, similar FIT rates may
be applied with device failure rates from the Telecordia SR-332
specification in order to sum a design topology FIT rate. The
example FIT rates applied across different components to create a
normalized calculation is shown in Table
1. Notice the optocoupler has the highest failure rate, also
note that acceleration factors have not been applied to Table 1 FIT
rates.

Table 1. Assumed Component Device Failure
Rates (FIT)

Applying these common values to the LED driver
BOMs, Table 2 shows the component
counts and final FIT values.

Table 2. LED Driver Comparisons

There are several important points to be
recognized from the data in Table 2. As the number of components
increases, the failure rate also increases — the axiom
“component count reduction increases
reliability” holds true. The addition of an electrolytic
capacitor will reduce the assembly reliability, but the
acceleration of component failures because of temperature, voltage
and current, or other stresses will affect the overall LED driver
reliability. Wear-out from solder joints is also not accounted,
however, based on the first pass assumption there are two solder
joints per component, so the FIT rates for the topologies in Table
2 would increase but not change in order of topology preference
because the component count is shown to be increasing.

The data in Table 2 does not take into account
significant quality, stress, and temperature factors. Quality is
related to the component’s packaging ability to eliminate
environmental causes of reduced reliability. For example, does the
electrolytic capacitor case resist moisture or expand/contract with
temperature variations? Electrical stress would include the
electrolytic capacitor ripple current and voltage ripple.
Temperature, for an LED driver, is one of the greatest accelerators
to reduce reliability because the typical residential bulb-type
applications are potentially in worse-case environments. The
fixture, once populated with an incandescent bulb with heat
radiating as IR energy, is replaced with a LED bulb that relies
more on conduction through the fixture to keep the temperature low
in the LED driver. This is especially true when air flow is not
present in a downward oriented fixture. The temperature stress
factor is dominant in most LED driver designs and is offset by the
use of higher rated temperature electrolytic capacitors or the
elimination of electrolytic capacitors all together – the
trade-off being capacitor cost.

The lowest FIT rate in Table 2 is for the
non-isolated buck topology with no electrolytic capacitors; the
Fairchild FL7701 is an example design controller shown in Fig. 1.[2]

Fig. 1: Fairchild FL7701 non-isolated buck
LED driver.

The FL7701 driver controller IC has PFC
integrated into its control strategy and the control IC can be
powered from the rectified off-line input using a small ceramic
capacitor for hold-up. This alone eliminates the need for an input
electrolytic capacitor and the electrolytic capacitor used for the
IC self-bias rail. With a LED load that does not have to meet tight
current ripple requirements, all electrolytic capacitors can be
eliminated, improving the warranty period of the driver. The ripple
current on the LEDs may decrease their lifetime but the trade-off
in reliability between the LED driver and the LEDs may result in a
better overall reliability figure for the fixture on the wholeThis
topology has the lowest BOM count and can be used without
electrolytic capacitors to have a significantly lower FIT rate,
almost a half to a third lower failure rate than other leading
topology choices for LED driver designs, as seen in Table 2. Using
a driver such as the FL7701 will reduce the MTBF value of a typical
LED driver circuit while adding an electrolytic capacitor will
still keep the failure rate less than other major topologies.

The calculations used in this article are not
meant to be final predictors for LED driver reliability.
Acceleration factors have not been applied so the actual LED driver
reliability requires testing of the drivers in an application
environment known as life testing. Nevertheless, reliability
calculations can be used to help a designer understand the
trade-off in topology selection or component selection. A
non-isolated buck topology offers a low component count which in
turn may result in good reliability ratings, if the ambient
temperature around the LED driver components can be managed
properly.